1. Field of the Invention
The present invention relates to a chip-type LED and a process of manufacturing the same, and more particularly, it relates to a chip-type LED utilized as a light source for various display panels or a backlight source for liquid crystal display devices.
2. Description of Related Art
There has been known a common chip-type LED comprising a pair of lead frames each extending to the bottom of a concave portion of a molded article, a LED element mounted on one of the lead frames and a light-transmissive resin filled in the concave portion. Some of the chip-type LEDs utilize an LED element which emits blue or bluish-purple light and a light-transmissive resin containing a fluorescent material.
The chip-type LED of this kind converts a part of the blue or bluish-purple light to yellow light and mixes the yellow light and the blue or bluish-purple light to emit white light (see Publication of Japanese Patent No. 2927279).
The molded article used in the above-mentioned chip-type LED is formed by insert molding, i.e., by injecting a resin into a mold supporting the pair of lead frames therein. In general, a modified polyimide resin is used as the resin.
The light-transmissive resin may generally be a light-transmissive epoxy resin which is resistant to heat generated by soldering.
In the common chip-type LED, the light-transmissive resin may be expanded when heat is externally applied by soldering or the like. This may cause a thermal stress to a junction between the molded article and the light-transmissive resin, the LED element itself and a gold wire connecting the LED element and the lead frames. In such a case, the light-transmissive resin may come off the molded article or the gold wire may be cut off.
As the cause of the above inconvenience, considered is a difference between expansion coefficient of the epoxy resin used as the light-transmissive resin for filling the concave portion and that of the modified polyimide resin used as a material for the molded article.
In general, the expansion coefficient of the epoxy resin is 5−8×10−5/° C., while that of the modified polyimide resin is 2−6×10−5/° C.
Where an epoxy resin containing a fluorescent material is used to fill the concave portion in order to emit white light, the fluorescent material precipitates at the bottom of the concave portion as time goes by even if it is sufficiently mixed with the epoxy resin in a liquid state. Such a problem is caused by the specific gravity of the inorganic fluorescent material that is greater than that of the organic epoxy resin.
When the fluorescent material precipitates at the bottom of the concave portion, a concentration of the fluorescent material becomes lower than a predetermined value at the upper portion of the concave portion, whereas it becomes higher at the bottom portion.
In such a case, there is caused a difference in distance across the fluorescent material covered by light emitted from the upper surface of the LED element and that covered by light emitted from the side surface thereof. Accordingly, a degree of conversion from the blue light or bluish-purple light to the yellow light is also varied. As a result, when the opening of the concave portion is viewed from the top, a portion of strong blue light and a portion of strong yellow light are observed, thereby generating unevenness in color.
Even if the fluorescent material concentration in the light-transmissive resin is uniform, it is still difficult to completely avoid the unevenness in color because there is a difference in distance across the fluorescent material covered by light emitted from the upper surface of the LED element and that covered by light emitted from the side surface thereof.
Further, when the LED element which emits ultraviolet light such as bluish-purple light is utilized, luminance is lowered and the resulting LED would not be suitable for practical use because the light-transmissive resin such as the epoxy resin is decomposed by the ultraviolet light and turned yellow or black.